1. Field of the Invention
The present invention relates to an inkjet recording apparatus and a recording medium conveyance method for the inkjet recording apparatus.
2. Description of the Related Art
Conventional inkjet recording apparatuses are provided with an upstream side conveyance unit that performs conveyance of a recording medium at an upstream side in the conveyance direction and a downstream side conveyance unit that performs conveyance of the recording medium at a downstream side in the conveyance direction.
Generally, the upstream and downstream conveyance units include a driving roller and a pinch roller that is arranged at a position facing the driving roller and is elastically urged to the driving roller by a pressing unit such as a spring.
The driving roller at the upstream side is also referred to as a conveyance roller. The conveyance roller is a metallic roller formed to have fine uneven shapes on the surface to produce a large frictional force. The driving roller at the downstream side is also referred to as a discharge roller. For the roller, generally, a roller that is formed of a material having a large frictional force such as rubber is used.
For the purpose to prevent slack of a recording medium, as compared to the conveyance roller, the diameter of the discharge roller is set to increase the speed by 0.3 to 1%. In the setting, the holding force at the downstream side conveyance unit is smaller than that at the upstream side conveyance unit. Accordingly, when the recording medium is nipped by the both units, the recording medium is conveyed in a state that slip is generated at the side of the downstream side conveyance unit.
In the inkjet recording apparatus, ink is perpendicularly discharged from a discharge port face of the inkjet head. However, due to various factors, the discharging direction may be tilted, and may cause deviation in ink impact positions. This deteriorates the image quality. In such a case, the shorter the distance (nozzle height) between the discharge port face and the recording medium is, the smaller the impact position deviation is. However, if the nozzle height is lowered, due to curl (distortion) or flotation of the recording medium, ruffling caused by ink absorption on the recording medium, and the like, the recording medium and the discharge port face may come in contact with each other. Then, the recording head may be broken, or the recording medium may be stained with the ink.
In order to regulate the flotation of the recording medium, a suction platen is employed. The suction platen is made by forming many suction holes on a recording medium contact surface of a suction box that is formed in a box shape. By exhausting air from the inside of the platen using a fan, negative pressure is produced. By the pressure, the recording medium can be attracted and held on the recording medium contact surface of the platen.
For example, Japanese Patent Application Laid-Open No. 2007-276292 discusses a control method, in which a humidity sensor is provided in a printer, and based on a detected humidity, a negative pressure produced via a platen is controlled to be changed. In this technique, in an environment where paper flotation is large, by controlling the negative pressure to increase, the flotation of the recording medium is reduced. Then, the negative pressure is not increased more than necessary, and the power of the apparatus can be saved and the noise can be reduced.
Now, the amount of the conveyance of the recording medium is described. Conventionally, it has been known that in a state in which a conveyance load (hereinafter, referred to as back tension) is not applied, a conveyance amount of a recording medium by a conveyance roller is equals to a length of the outer circumference surface of the conveyance roller used for the conveyance.
According to the theory, if the conveyance amount of the recording medium is Lm, the outside diameter of the conveyance roller is D, and the rotation amount of the conveyance roller is θ [deg], the conveyance amount Lm of the recording medium can be expressed as follows:Lm=π×D×(θ/360)
When back tension is applied by the structure of the recording apparatus, between the conveyance roller and the recording medium, slip is produced. Then, the conveyance amount Lm of the recording medium may be shorter than the length of the outer circumference surface of the conveyance roller used for the conveyance. The relationship can be expressed as follows:Lm≦π×D×(θ/360)  (1)
It has been considered that the amount of the decrease of the conveyance amount Lm differs depending on the types of the recording media. As described above, it has been known that the conveyance amount of the recording medium is to be a movement amount equivalent to the outer diameter of the conveyance roller or slightly decreased by the slip, and the degree of the decrease differs depending on the types of the recording media.
In order to prevent the deterioration in the recording quality due to the deviation (hereinafter, referred to as conveyance error) of the conveyance amount of the recording medium, Japanese Patent No. 03070234 discusses a technique to change a feed amount of a conveyance roller using a feed correction value for each type of recording media.
By the technique to correct the conveyance amount to an optimum value depending on the types of the recording media, it is possible to correct a certain amount of conveyance error.
However, in the method for correcting the conveyance amount for each recording medium, even if the type of the recording medium is the same, if negative pressure (hereinafter, referred to as platen suction pressure) of the suction platen differs, the recording medium is not conveyed at a constant conveyance amount. Then, streak is produced on the recording image.
Especially, when the recording medium is conveyed by only the discharge roller of the downstream side that has a low holding force and the recording operation is performed, the platen suction pressure largely affects the conveyance error, and results in image deterioration.